A steel sheet for vitreous enameling was conventionally produced by applying an annealing treatment for decarbonization and denitrification and lowering C and N contained therein to several tens of ppm or less. However, such an annealing treatment for decarbonization and denitrification had the drawbacks of low productivity and a high production cost. As a technology for avoiding an annealing treatment for decarbonization and denitrification, Japanese Unexamined Patent Publication No. H6-122938 discloses a steel sheet for vitreous enameling, the steel sheet being produced from ultra-low carbon steel obtained by lowering the C content to several tens of ppm through degassing in a steelmaking process. In such a technology, drawability and aging resistance are improved by adding Ti, Nb, etc. to avoid the adverse effects of solute C or solute N that still remains slightly in a steel sheet. However, the problems of the technology are that the defects such as bubbles and black spots are likely to be caused by carbides and nitrides and the production cost increases due to the addition of Ti, Nb, etc.
As technologies of solving the problems, steel sheets for vitreous enameling, wherein the addition amounts of Ti, Nb, etc. are decreased, though drawability deteriorates to some extent, and methods for producing the steel sheets have been invented and are disclosed in Japanese Unexamined Patent Publication Nos. H8-27522 and H10-102222 and other publications. These technologies are ones wherein B is used predominantly for fixing N. However, the problems of the aforementioned disclosed technologies are: that aging properties deteriorate and thus formability in pressing is impaired since the lowering of solute C is insufficient sometimes depending on the production conditions and N increases caused by the re-melting of nitrides during annealing; and that the defects such as bubbles and black spots are likely to be caused by the gases generated by the decomposition of nitrides and the like during the baking of a vitreous enamel.
The object of the present invention is to overcome the above-mentioned problems of a conventional steel sheet for vitreous enameling, to provide a non-aging and low-cost steel sheet for vitreous enameling, the steel sheet being excellent in resistance to bubbles and black spots, and to provide a method for producing the steel sheet.